Lets Talk: A Story of Interspecies Communication

It was Sept 4, 1939, the day after the UK declared war on Germany, when mathematician Alan Turing reported to work at the Government Code and Cypher School at Bletchley Park. Within weeks of his arrival, Turing and his colleagues were able to intercept high-level encrypted enemy communication signals and decode a vast number of these messages. The intelligence gleaned from this effort was passed on to field commanders, a process that was decisive to Allied victory.

Like the German military strategists, single-celled bacteria communicate with each other using coded messages to coordinate attacks on their targets. For bacteria these targets are plants and animals that provide the nutrients needed for growth. Until now, the diversity of codes employed by invading bacteria was thought to be extremely limited. However, our new research shows that bacteria communicate with a previously unknown signal. The research is described in two articles published today in the Public Library of Science and Discovery Medicine.

In a feat worthy of the Turing cryptographers, some plants have evolved a cypher-breaking detection system, called the XA21 receptor, that intercept the bacterial code and use this information to trigger a robust immune response, preventing disease.

Over the last 20 years, researchers have shown that bacteria employ specific signals to communicate. These signaling molecules accumulate in the external environment as the cells grow. When the concentration of signal reaches a certain threshold level, the individual bacteria mobilize concerted, group actions. Professor Bonnie Bassler, an early pioneer in studies of bacterial communication, calls the signaling molecules “bacterial Esperanto”.

Until now, it was thought that two major groups of bacteria (called Gram-positive and Gram-negative bacteria) use distinctly different types of communication codes. Gram-positive bacteria use oligopeptides, whereas Gram-negative bacteria generally use acylated homoserine lactones (AHLs) or diffusible signal factors (DSF). However, the newly discovered signal, called Ax21 (Activator of Ax21-mediated immunity), from the Gram-negative infectious bacterium Xanthomonas oryzae pv. oryzae), falls into neither class.

Unlike other signals in the bacterial coding repertoire, Ax21 is a small protein. It is made inside the bacterial cell, processed to generate a shorter signal and then secreted outside the bacterium. Perception of Ax21 by other bacteria of the same class, allows the bacteria to assemble into elaborate protective bunkers, called biofilms. Biofilms render the bacteria resistant to dessication and antibiotic treatment. Thus, by virtue of communication and communal living, bacteria increase their chances of survival and proliferation. Ax21 perception also regulates the production of a virulent arsenal including “effectors” that are shot directly into the host to disrupt host defenses and the initiation of motility allowing the bacteria to colonize new sites for infection.

This process transforms the bacteria from a benign organism to a fierce invader. The bacteria multiply in the main arteries of the rice water transport system, causing the plant to wither and die.

To accomplish these diverse tasks, Ax21 perception triggers a massive change in the genetic program: Nearly 500 genes (approximately 10% of Xoo genome) change their expression in response to Ax21. Bacterial mutants defective in Ax21 no longer aggregate into bunkers, move to new sites or trigger changes in gene expression.

Host cryptographer: The XA21 receptor

Most rice plants are virtually defenseless to this Ax21-mediated bacterial attack. The exceptions are those plants that carry the XA21 immune receptor that detect Ax21 produced by the invading microbe.

[Bruce and I share more than an interest in science; my father (Robert Rosenthal) and Bruce’s father (Ernst) were young cousins in Berlin in the 1920s. Their families fled the Nazi’s and reunited in the US after the war. I was honored to hear Bruce discuss XA21/Ax21 and our shared family history during his Nobel lecture last week (starts at 40:45)]

In plants and higher animals, these immune receptors detect microbial components that are conserved among diverse bacteria. These include structures that make up the bacterial cell wall or are important for motility.

The advance that we reported today indicates that not only does this class of receptors recognize structural components of bacteria but that they can also detect bacterial signaling molecules. The ability of plants to intercept these messages provides them with a clear tactical advantage in the evolutionary battle. To date, only the XA21 immune receptor is known to have this capacity.

Early detection of the signal produced by the invading bacteria is critical because it allows the plant time to mobilize defenses. Thus, just as the work of Turing allowed Allied convoys to detect and evade U-boat patrol lines, and then guide Allied anti-submarine forces to destroy the U-boats, XA21 intercepts the Ax21 signal, allowing rice to mount an early and potent defense response.

Ax21 is present in other pathogens of plants and animals

We have shown that not only is Ax21 present in important plant pathogens such as Xanthomonas, which infects virtually all crop plants and in a microbe that causes Pierce’s disease on grapes, but it is also present in a human pathogen that infects hospital patients, such as those suffering from cystic fibrosis.

The conservation of Ax21 in both plant and animal pathogens suggests that Ax21 also serves as a signal in these related microbes. In support of this idea, some of the functions of Ax21 discovered in the rice pathogen have been recently extended to pathogens of peppers, tomatoes and mustards as well as to a human pathogen.

The discovery that a small protein from a group of single-cell bacteria plays a dual role in both rallying invading bacteria and triggering an immune response in the targeted plant, has not previously been demonstrated. However, exploration of bacterial genomes predicts the presence of an abundance of similar small secreted proteins and their predicted secretion systems in many other species. These discoveries suggest the intriguing possibility that other species of bacteria use small proteins like Ax21 to communicate and coordinate infection.

The new research also suggests that rice is not the only targeted victim that has learned to detect these abundant bacterial signals. Whereas only 10 immune receptors have been identified in humans, over 300 such receptors are predicted in rice and other important cereals. Unlike rice XA21, which has been shown to bind directly to Ax21, few corresponding conserved microbial signature has yet been identified for the hundreds of other predicted rice immune receptors. Thus, the plant immune response remains largely unexplored. An important question for future research is to identify the microbial molecules that these “orphan” receptors detect. Because most bacteria are in constant communication, it is clear that bacterial signals will accumulate in the host vicinity prior to infection. We speculate that some of the other hundreds of predicted receptors may have also evolved to intercept these bacterial messages.

For example, as described today in the journal Discovery Medicine, it may be possible to develop drugs that can antagonize Ax21-mediated biofilm formation, a process thought to occur in 65-80% of bacterial infections of plants and animals.

#, Present address: Department of Chemical and Process Engineering, Thai-German Graduate School of Engineering, King Mongkut’s University of Technology North Bangkok, Bangkok, Thailand

Research on Ax21 was supported by National Research Initiative grants 006-01888 and 2007-35319-18397 from the USDA Cooperative State Research, Education and Extension Service. The discovery of XA21 was supported by the National Institute of Health grant # GM59962 to PCR.

Pamela Ronald is Professor of Plant Pathology and Chair of the Plant Genomics Program at the University of California, Davis, where she studies the role that genes play in a plant’s response to its environment. With her husband, she co-wrote Tomorrow's Table: Organic Farming, Genetics and the Future of Food. Dr. Ronald was one of the co-founders of Biology Fortified.